The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
In the offset printing art, printing plates are almost exclusively made with aluminum supports. Inherently, aluminum is a relatively soft metal so that it is frequently the case that the printing plate is subject to scratching or other damage in use.
Aluminum has been used for many years as a support for lithographic printing plates. In order to prepare the aluminum for such use, it is typical to subject it to both a graining process and a subsequent anodizing process. The graining process serves to improve the adhesion of the subsequently applied radiation-sensitive coating and to enhance the water-receptive characteristics of the background areas of the printing plate. The graining affects both the performance and the durability of the printing plate, and the quality of the graining is a critical factor determining the overall quality of the printing plate. A fine, uniform grain that is free of pits is essential to provide the highest quality performance.
Both mechanical and electrolytic graining processes are well known and widely used in the manufacture of lithographic printing plates. Optimum results are usually achieved through the use of electrolytic graining, which is also referred to in the art as electrochemical graining or electrochemical roughening, and there have been a great many different processes of electrolytic graining proposed for use in lithographic printing plate manufacturing. Processes of electrolytic graining are described, for example, in U.S. Pat. Nos. 3,755,116, 3,887,447, 3,935,080, 4,087,341, 4,201,836, 4,272,342, 4,294,672, 4,301,229, 4,396,468, 4,427,500, 4,468,295, 4,476,006, 4,482,434, 4,545,875, 4,548,683, 4,564,429, 4,581,996, 4,618,405, 4,735,696, 4,897,168 and 4,919,774.
Use of electrochemical graining requires the use of aluminum which is very pure and therefore very soft and this further aggravates the problem of scratch formation.
In the manufacture of lithographic printing plates, the graining process is typically followed by an anodizing process, utilizing an acid such as sulfuric or phosphoric acid, and the anodizing process is typically followed by a process which renders the surface hydrophilic such as a process of thermal silication or electrosilication. The anodization step serves to provide an anodic oxide layer and is preferably controlled to create a layer of at least 0.3 g/m.sup.2. Processes for anodizing aluminum to form an anodic oxide coating and then hydrophilizing the anodized surface by techniques such as silication are very well known in the art, and need not be further described herein.
Included among the many patents relating to processes for anodization of lithographic printing plates are U.S. Pat. Nos. 2,594,289, 2,703,781, 3,227,639, 3,511,661, 3,804,731, 3,915,811, 3,988,217, 4,022,670, 4,115,211, 4,229,266 and 4,647,346. Illustrative of the many materials useful in forming hydrophilic barrier layers are polyvinyl phosphonic acid, polyacrylic acid, polyacrylamide, silicates, zirconates and titanates. Included among the many patents relating to hydrophilic barrier layers utilized in lithographic printing plates are U.S. Pat. Nos. 2,714,066, 3,181,461, 3,220,832, 3,265,504, 3,276,868, 3,549,365, 4,090,880, 4,153,461, 4,376,914, 4,383,987, 4,399,021, 4,427,765, 4,427,766, 4,448,647, 4,452,674, 4,458,005, 4,492,616, 4,578,156, 4,689,272, 4,935,332 and European Patent No. 190,643.
The anodization process is intended to make the surface more resistant to wear and to provide enhanced adhesion for the light-sensitive coatings that are applied thereto, but the oxide layer formed thereby is very thin and therefore easily subject to damage. Moreover, the hardness of the oxide layer is dependent on the particular characteristics of the anodization process utilized and the softer it is the more prone it is to damage from scratches.
Due to the environment in most print shops, it is unlikely that a printing plate can ever be robust enough to withstand the diverse conditions and methods of handling. Quite often, a plate is scratched before it gets to press. If the scratch is light and has not broken through the oxide layer, or has occurred on the image area, the print quality will not be affected. Many times, however, the oxide layer is seriously damaged and the area of damage will become ink receptive. Pressmen try various approaches to render these damaged areas hydrophilic, but typically such attempts are ineffective or short lived. Manufacturers of printing plates, as well as those producing ancillary chemicals for printers, commonly manufacture scratch remover compositions intended to restore hydrophilicity as an extended or permanent correction. Typically, these compositions are incapable of performing in a fully acceptable manner. The aim has been to formulate a composition that is easy to use and will effectively desensitize the damaged area under a variety of conditions so that a pressman will have a high likelihood of being able to use the plate in a normal manner and not have to replace it or experience excessive press stoppage for extensive corrective treatment. This has proven to be extremely difficult to achieve.
Many compositions have been proposed for use as scratch removers and desensitizers for lithographic printing plates and/or for such related functions as plate cleaners and plate finishers.
Examples include desensitizer compositions containing silicates, wetting agents and hydrophilic colloids as described in U.S. Pat. No. 4,258,122, issued Mar. 24, 1981; fountain solutions comprising trisodium phosphate, sodium metasilicate, tetrapotassium pyrophosphate, a nonionic surfactant and a dialkylpolysiloxane as described in U.S. Pat. No. 4,340,509, issued Jul. 20, 1982; scratch remover compositions comprising a water-in-oil emulsion as described in U.S. Pat. No. 4,399,243, issued Aug. 16, 1983; plate cleaning compositions comprising a silicate and a cationic or amphoteric surfactant as described in U.S. Pat. No. 4,576,743, issued Mar. 18, 1986; scratch remover compositions comprising trisodium phosphate, sodium metasilicate and an anionic surfactant as described in U.S. Pat. No. 4,778,616, issued Oct. 18, 1988, and plate cleaning compositions comprising an organic solvent, sodium metasilicate and a nonionic surfactant as described in U.S. Pat. Nos. 4,886,553, issued Dec. 12, 1989 and 4,997,588, issued Mar. 5, 1991.
It is toward the objective of providing a new and improved scratch remover and desensitizer composition that overcomes the disadvantages of prior art compositions, and more effectively meets the needs of the lithographic printing plate art, that the present invention is directed.